Referring to FIG. 5, an electroacoustic transducer includes, as a basic configuration, a diaphragm 10, a magnetic circuit part 20, and a unit frame 30 for supporting these elements.
The diaphragm 10 has a center dome 11 and a sub dome 12 integrally provided around the center dome 11, and the entire thereof consists of a thin film made of a synthetic resin. On the back surface side of the diaphragm 10, a voice coil 13 is fixed to the boundary portion between the center dome 11 and the sub dome 12 with an adhesive. In some cases, the sub dome 12 is also called an edge.
The magnetic circuit part 20 includes a yoke 21 formed in a dish shape, a disc-shaped permanent magnet 22 disposed in the center of the yoke 21, and a center pole piece 23 disposed over the permanent magnet 22. The permanent magnet 22 is magnetized in the plate thickness direction, and thereby a magnetic gap G is formed between the center pole piece 23 and the opening end portion of the yoke 21.
The unit frame 30 consists of a disc body having a cylinder part 31 in which the magnetic circuit part 20 is fitted in the central portion thereof, and in the outer peripheral portion thereof, a rib 32 for positioning the diaphragm 10 is formed in a ring form. Also, on the inside of the rib 32, a flat support face 33 is formed.
Referring to FIGS. 6 and 7, at the peripheral edge of the sub dome 12, a flat flange part 12a is provided. The diaphragm 10 is configured so that the flange part 12a is fixed to the support face 33 with an adhesive 40 in such a manner that the voice coil 13 can oscillate in the magnetic gap G.
In the case of a headphone and a speaker (electroacoustic transducers), the voice coil 13 functions as a driving coil for oscillating the diaphragm 10 by sound signals applied from the outside, and sounds are discharged from the diaphragm 10. In the case of a microphone (electroacoustic transducer), the voice coil 13 functions as a power generating coil, and sound signals are generated from the voice coil 13.
In both the cases, when the control system is mass control, the low frequency limit capable of capturing or reproducing sounds is determined by the resonance frequency of low frequency region. For this reason, the sub dome 12 is designed so that the stiffness of the diaphragm 10 is low.
However, since the mechanical strength of the sub dome 12 is low, when the flange part 12a is bonded to the unit frame 30, as shown in FIG. 7, internal stresses F1 and F2 remain sometimes in the sub dome 12 on account of the shrinkage of the adhesive 40 or if the bonding is performed in a state in which a stress is applied to the flange part 12a. The internal stresses F1 and F2 are liable to occur especially in a portion where the adhesive 40 squeezes out.
By such internal stresses, the piston-like movement of the entire of the diaphragm 10 is restrained. For the headphone, so-called chattering noise (abnormal resonance) is generated, and for the microphone, the frequency response in the middle and high frequency regions is deteriorated. Also, troublesomely, the internal stresses are not generated uniformly, and vary between individual portions.
To solve these problems, as the adhesive, an ultraviolet-curing resin that shows elasticity even after curing is preferably used, and after the diaphragm 10 has been placed on the unit frame 30 in such a manner that the internal stresses do not remain, the flange part 12a is bonded to the unit frame 30 with the aforementioned ultraviolet-curing resin. Also, in the invention described in Japanese Utility Model No. 2548580, a viscous liquid that does not cure is interposed between the flange part 12a and the support face 33.
Thus, the configuration is made such that the abnormal resonance of the sub dome 12 is prevented by mechanically braking the bonding portion of the sub dome 12. However, the mechanical impedance in the bonding portion still increases suddenly as viewed from the sub dome 12 side. Therefore, end reflected waves (the arrow mark B in FIG. 5), which is caused by the reflection of traveling waves (the arrow mark A in FIG. 5) propagated from the center dome 11 side on the end face of the bonding portion, are liable to occur, and therefore the abnormal resonance cannot be prevented reliably.
Accordingly, an object of the present invention is to provide an electroacoustic transducer in which abnormal resonance caused by end reflected waves in the bonding portion of a diaphragm and a unit frame is prevented.